It is known that magnetooptical disk memories are memories in which the data are recorded on magnetic disks and read with optoelectronic devices. These memories are being used to an ever increasing extent, because they make it possible to obtain high radial and longitudinal densities, on the order of several thousand tracks per cm radially and 100,000 data per cm longitudinally. (The radial density is the number of tracks per unit of length, measured along the diameter of the disk, while the longitudinal density is the number of data per unit of length measured along the circumference of a track.)
The manner in which magnetooptical disk memories function is based on the magnetooptical effect, which relates to the interaction of a light, polarized rectilinearly, with the magnetic state of the material comprising the recording layer of the magnetic disks. Further details on the magnetooptic effect and on the manner in which data contained on magnetic disks in a magnetooptical memory can be read are disclosed in U.S. Pat. No. 4,510,544 assigned to the assignee of the present invention.
For the sake of simplicity, the means that enable either writing or recording of the data on the disks or reading thereof are called transducers or sensors. Typically one or more transducers (or sensors) are linked with one face of a given disk, and the disk revolves past the transducer or transducers.
One of the current trends in the development of magneto optical disk memories is toward memories in which the data are written on the magnetic disks by magnetic transducers, the reading being performed by an optoelectronic device including a rather complex optical focusing device linked to photoelectronic transducers and circuits for amplifying the signals emitted by these transducers. With these optoelectronic devices, it is thus possible to observe one face of a disk by means of a beam of polarized light at a given moment, in a given site, and to emit an electrical signal the voltage (or current) of which is a function of the data located at this site.
Magnetic transducers used for writing the data are described in French Patent Application 84 200 25, filed on Dec. 28, 1984 and corresponding U.S. Patent Application, Ser. No. 813,236, both of which are assigned to the assignee of the present invention.
U.S. Pat. No. 4,633,450, assigned to the assignee of the present invention describes a magnetic writing and optical reading head for magnetooptical disk units in which the writing is performed by a magnetic transducer, and the reading is performed by an optical focusing device placed inside a recess provided inside the head. This head is called the "main body" and is of the low-charge Winchester type and is, for example, comprised by a catamaran including at least two runners (also known as rails or skis, in the terminology used by those skilled in the art) and a groove disposed between them.
The focusing optical element is placed in a recess provided inside one of the two runners. This reading and writing head flies above the data carrier at a distance, known as the flight altitude, which is on the order of several tenths of a micrometer. Hence the focusing optical element is located at a distance from the data carrier that is greater than or equal to this flight altitude. Thus because of the same fact of the conditions of flight of the head above the disk, which keep the face of the head that faces the disk at a constant distance from it, it is understood that in suitably disposing the focusing optical element inside the head, the distance between this optical element and the disk can be made constant during the entire time of the flight of the head above the disk.
It is thus possible to avoid using an extremely heavy and expensive device for automatic focusing.
In a preferred embodiment of the above-described head, the magnetic transducer and the focusing device (which is also known as an optical sensor) are located on the same rail.
The head is mounted at the end of a suspension arm, which in turn is mechanically connected to the movable portion of a linear or rotary actuator. To make magnetooptic disk memories less bulky, it is preferable to use rotary actuators, because they need less electrical energy than a linear actuator, and they are smaller in size. A rotary actuator of this kind is described in U.S. Pat. No. 4,571,648, assigned to the assignee of the present invention, and includes an assembly that is movable to rotate about an axis that is parallel to the axis of rotation of the magnetic disk and is located outside the disk. When the rotary actuator is displaced, the head structurally connected with the support arm is displaced above the disk, describing a circular arc.
When the reading and writing head is located above any one of the tracks of the disk, its longitudinal axis forms an angle .psi. with the tangent to the track. This angle, known as the azimuth angle, varies depending on the track above which the head is located. Thus if the head is displaced from the outside track located on the periphery of the disk to the inside track closest to the center of the disk, this angle .psi. can vary, for example between -7 and +7.degree.. It will be understood that the variations of this angle are a function of geometrical arrangements relating to the disk and to the assembly formed by the rotary actuator, the suspension arm and the head. Since by construction the magnetic transducer and the focusing device have a common longitudinal axis of symmetry and this axis is parallel to the longitudinal axis of the flying head, the following conditions obtain:
For typical distances separating the optical axis of the optical focusing device and the axis of symmetry of the writing pole of the magnetic transducer (perpendicular to the track), that is, several tenths of a millimeter to one millimeter, there is a positional spacing with respect to the tracks of the disk, between the magnetic writing transducer and the optical focusing device. Thus if the writing pole of the magnetic transducer is located facing a track having the ordinal number i, the focusing optical element is located facing the track having the ordinal number j, where i is different from j. For a distance between the optical axis of the optical focusing device and the writing pole of the transducer on the order of a millimeter, this positional spacing (i -j) is on the order of several tens of tracks.
This entails the following disadvantage:
It is not possible to reread the data written by the magnetic transducer using the optical reading device including the focusing optical element immediately after these data have been written, when this operation is currently performed in the classical disk memories, which typically and as is well known include only a single magnetic transducer for both reading and writing. Thus all the data must first be written on one track by means of the transducer and then once these writing operations are terminated, the writing and reading head must be displaced in such a manner that the optical focusing device is located facing the track which has just been written upon. In this case, two position control systems must be used, one for the writing operations and the other for the reading operations. Each of these devices has its own device for measurement of position (in fact, if only the single optical device including the focusing optical element is used for measuring the position of the head above the disk, then it is certainly possible to know the position of this focusing optical element with respect to the disk, but the position of the magnetic writing transducer will not be known, since the positional spacing between the magnetic transducer and the focusing optical element with respect to the tracks varies, depending on the position occupied by the head above the disk). Using two position control systems is expensive and complicated. Moreover, the operations of writing data and the operations of rereading these data are obviously relatively lengthy, in any case much lengthier than the operations of writing and rereading data in classical disk memories.